cosmian kyber is a fork of Argyle-Software/kyber and inherits a broken constant-time conditional-move routine cmov in its AVX2 backend. When built with the opt-in avx2 feature on x86 64, cmov is a no-op: it never performs the copy. That cmov is the step that applies the Fujisaki-Okamoto (FO) implicit rejection during decapsulation. With it disabled, decapsulating an invalid ciphertext no longer returns a key-independent pseudorandom value; it returns a value that depends on the decrypted plaintext of the attacker-chosen ciphertext. This restores the chosen-ciphertext decryption oracle that the FO transform exists to remove, and yields full recovery of the static secret key.

The attack requires no timing measurement, no side channel, and no faults; only the ability to submit ciphertexts to a decapsulation operation under a reused key pair and observe the resulting shared secret. See the linked pull request for the root cause and the reachable call path.

Reachable only in builds with features = ["avx2"] on an x86 64 target. The default (reference) backend and all non-x86 64 targets are unaffected.

Complete IND-CCA break. The plaintext-checking oracle drives a standard chosen-ciphertext key-recovery attack and extracts the entire static secret key. Any protocol reusing a Kyber key pair across decapsulations is exposed; ephemeral-only key shares are not. The identical defect in the parent crate was verified end to end on ML-KEM-768 (full key recovered in 4,272 decapsulation queries).

cosmian kyber is a stale fork (last published 0.1.0, January 2023) of an unmaintained upstream, and no fixed release is expected.

Reported by 007bsd.